A sodium-proton exchange mechanism is present in the plasma membrane of most cells and is involved in the regulation of intracellular pH, cell volume and cell growth. In the NHE family of ion exchangers there are at least eight isoforms of NHE (NHE-1, NHE-2, NHE-3, NHE-4, NHE-5, NHE-6, NHE-7, NHE-8) known to date. NHE-1 is the ubiquitously expressed isoform and is called the housekeeping isoform, regulating intracellular pH, cell volume, cell proliferation, besides serving as the membrane anchor for the actin based cytoskeleton (Annu. Rev. Pharmacol. Toxicol. 42, 527-552, (2002)). It is a 110-kDa glycoprotein and constitutes the major isoform found in the mammalian myocardium. The other isoforms have a more restricted tissue distribution. NHE-2 and NHE-3 are confined to membranes of the renal and gastrointestinal tracts while NHE-4 is expressed in the gastrointestinal tract, and kidney. NHE-5 is expressed in the brain and kidney and may be involved in the regulation of cell pH and volume, while NHE-6 is a mitochondrial isoform regulating intra mitochondrial Na+, H+ and Ca++ levels (Annu. Rev. Pharmacol. Toxicol. 42, 527-552, (2002)). NHE-6 and NHE-7, which share only about 20% amino acid homology with other isoforms, are expressed in membranes of intracellular organelles (Clin. Invest. Med., vol. 25, no. 6, 229-232, (2002)). The most recently identified and cloned isoform NHE-8 is expressed in the kidney and is a candidate to mediate apical membrane ion transport in the proximal tubule (Am. J. Physiol. Renal Physiol. 284, F467-F473, (2003)). The NHE-1 isoform plays a major role in the regulation of cell volume and intracellular pH (Annu. Rev. Pharmacol., 42, 527-552, (2002)) and any changes in these cell parameters activate the NHE which cause transmembrane Na+ and H+ fluxes along the ionic gradients to regain homeostatic conditions. During ischemia, intracellular pH falls due to accumulation of protons, and subsequent NHE activation mediated proton efflux is associated with intracellular Na+ accumulation. Excess Na+ within the cell cannot be extruded because of depressed Na+/K+ adenosine triphosphate (ATP) ase activity and reduction in the transmembrane Na+ gradient will result in increased intracellular Ca++ levels via the Na+/Ca+ exchange mechanism acting in reverse mode. Intracellular Ca++ overload ultimately kills the cell. The loss of intracellular ATP during ischemia also results in phospholipase and protease activation causing cell membrane injury leading to necrosis (Clin. Exp. Pharmacol. Physiol, 27, 727-733, (2000)).
Currently, an unmet medical need exists for an agent that can minimize cardiac injury occurring both due to ischemia before thrombolytic therapy or coronary intervention and reperfusion thereafter. NHE activation leading to an intracellular Ca++ overload has been pathophysiologically linked to ischemic injury subsequently leading to arrhythmias, myocardial infarction and sudden death. In the light of the limitations of class I and class III anti-arrhythmic agents in treating such arrhythmias, this alternative approach via NHE inhibition is very promising. A chronic treatment using NHE inhibitors, given to patients suffering from acute myocardial infarction, may be helpful in minimizing injury due to subsequent ischemic episodes. Those with predisposing factors like obesity, diabetes and raised blood pressure are more likely to suffer from acute myocardial infarction and a prophylactic treatment using NHE inhibitors may expand the myocardial survival time till reperfusion can be restored. NHE inhibition can also retard gradual progression of angina pectoris into congestive heart failure (resulting from formation of microinfarcts) and improve post reperfusion cardiac performance (Cardiovasc. Res. 29, 189-193, (1995)).
In pre-clinical experiments, inhibition of cardiac NHE has been shown to significantly minimize the associated damage and arrhythmias. (Clin. Expt. Pharmacol. Physiol, 27, 727-733, (2000)). NHE inhibitors have been shown to offer cardioprotection in many animal models of ischemia and reperfusion injury and hence have good clinical potential in applications such as cardiac surgeries-Coronary Artery Bypass Grafting (CABG), Percutaneous Transluminal Coronary Angioplasty (PTCA), valve surgery, cardiac transplantation AMI and angina pectoris. (Cardiovascular Res., 29, 184-188, (1995)). They improve post ischemia/reperfusion cardiac performance and reduce myocardial infarction in animals (Clin. Expt. Pharmacol. Physiol., 27, 727-733, (2000)) and also in man (Circulation, 102 (suppl. III):III-319-III-325, (2000)). These compounds, when added into cardioplegic and organ preserving solutions, have been found to reduce ischemia/reperfusion-induced Ca++ overload and improve post surgical (e.g. CABG) or post transplant cardiac performance (Circulation, 102 (suppl. III):III-319-III-325, (2000)). The protective effect of NHE inhibitors against ischemia and reperfusion injury is evident for many other organs such as the brain, lungs and the skeletal muscle. Preoperative administration of NHE inhibitors is expected to reduce the incidence, extent and progression of cardiac injury and improve postoperative and post ischemia/reperfusion myocardial performance. This treatment is also expected to decrease morbidity and mortality and improve the quality of life in the ‘high risk’ group of patients prone to cardiac dysfunction.
Amiloride, a potassium sparing diuretic, was the first NHE inhibitor to be studied in ischemic isolated rat hearts (Circulation Res. 66:1156-1159, (1990)). Due to its low potency and specificity, the anti-arrhythmic activity of amiloride was found to be associated with undesirable side effects (Journal of Cardiovasc. Pharmacology, 17, 879-888, (1991)). Better potency, specificity and NHE-1 selectivity of the N-5 substituted derivatives of amiloride developed later, was not enough for eventual therapeutic development.
Like Amiloride, most of the known NHE inhibitors belong to the acylguanidine class of compounds, although in recent times non-acylguanidine NHE inhibitors have also been reported (WO 99/01435). Monocyclic acylguanidine NHE inhibitors are described in U.S. Pat. No. 5,591,754 (benzoylguanidines) and U.S. Pat. No. 5,700,839 (alkyl-5-methylsulfonylbenzoylguanidines). Bicyclic acylguanidine NHE inhibitors are described in U.S. Pat. No. 6,028,069 (heterocyclyl-condensed benzoylguanidines) and WO 00/64445. Tricyclic acylguanidine NHE inhibitors are described in WO 03/066620 (dihydrothiaphenanthrenecarbonylguanidines).
Acylguanidine derivatives as NHE inhibitors have also been described in the literature, see for example; Jpn. J. Pharmacol., 80:295-302, (1999); Eur. J. Pharm., 419, 93-97, (2001).
A variety of diseases are characterized by intracellular changes leading to harmful/undesirable effects following NHE activation caused by a variety of stimuli, such as ischemia, hypoxia, hypertonicity, hormones, mitogens and growth promoters (Cell Physiol. and Biochem., 11, 1-18, (2001)). Hypoxia/ischemia induced NHE activation ultimately leads to intracellular loading of Ca++ and cell death (Circulation, 102 (suppl III): III-319-III-325, (2000)). NHE expression has been linked to the development of malignant transformation. Neuronal pHi and osmoregulation, both closely regulated by NHE, have been shown to be disturbed during abnormal neuronal activity in depression and epileptic seizures. NHE-1 activation induced cellular swelling has been shown to be a prerequisite for directed migration of polymorphonuclear neutrophils in immune defense. NHE-1 may play an important homeostatic role in Airway Surface Liquid (ASL) osmolarity indispensable for normal lung function (Cell Physiol Biochem., 11, 1-8, (2001)). Metabolic acidosis, high sodium intake and raised levels of circulating insulin have been shown to be associated with NHE activation (Hypertension, 26, 649-655, (1995)). Cardioprotection against ischemia/reperfusion induced injury has been shown to be associated with inhibition of neutrophil activity (Am. J. Physiol.-Heart Circ. Physiol. 279: 4, H1563-H1570, (2000)). NHE-1 activity is increased in various cell types in hypertension and Type 1 diabetes (Diab. Nutr. Metab., 14, 225-233, (2001)). NHE-1 inhibition can prevent multifactorial hypertrophy of cardiomyocytes and reduce heart failure in vivo, independent of reduction in infarct size (Basic Res. Cardiol., 96, 325-328, (2001)).
There is a need for improved and alternative medicaments for the prevention and treatment of diseases associated with Na+/H+ exchange activity.